Iron cobalt vanadium alloy

ABSTRACT

DESCRIBED HEREIN IS A NOVEL AND IMPROVED ALLOY CONTAINING COBALT, IRON AND VANADIUM AND CONTROLLED QUANTITIES OF CARBON.

United States Patent O'flice 3,695,944 Patented Oct. 3, 1972 3,695,944 IRON COBALT VANADIUM ALLOY Carl P. Stroble, Natrona Heights, Pa., assignor to Allegheny Ludlum Industries, Inc, Pittsburgh, Pa. No Drawing. Filed June 17, 1970, Ser. No. 47,126 Int. Cl. H01f 1/14 U.S. Cl. 148-3155 2 Claims ABSTRACT OF THE DISCLOSURE Described herein is a novel and improved alloy containing cobalt, iron and vanadium and controlled quantities of carbon.

This invention relates to an improved alloy useful in electrical applications because of its magnetic properties. More particularly, the invention concerns an improved version of the cobalt, vanadium, iron alloy known commercially as, Vanadium Permendur. This alloy contains 47.5 to 50.5 cobalt, 1.7 to 2.1% vanadium and the balance essentially iron.

The vanadium, cobalt and iron-containing alloy is finding increasing use as a rotor and stator material in electrical generators for aircraft because of its high magnetic flux carrying capacity which makes it possible to achieve a considerable reduction in weight of the units in which components of the alloy are employed. As a result of the high operating speeds, i.e., 8,000 to 20,000 rpm. of these generators, it is also important that the material have high mechanical strength as well. Although specific requirements may vary, some applications require a minimum .2% ofiset yield strength of 70,000 p.s.i. coupled with the minimum tensile elongation of 5% and relatively little deterioration in magnetic properties. Unfortunately, the presently commercial version of this alloy develops a yield strength of only approximately 55,000 p.s.i. when annealed for four hours at 1385 F., which is a standard pilot test procedure.

The present invention provides a composition which possesses significantly improved strength while retaining satisfactory magnetic properties. In accordance with this invention, there is provided an alloy consisting essentially of 47.5 to 50.5% cobalt, 1.7 to 2.1% vanadium, a controlled carbon content of 0.025 to 0.1%, preferably 0.03 to 0.08%, and the balance essentially iron. Larninations or strips made of alloys in accordance with the invention possess increased yield strength and relative insensitivity to normal variations in annealing conditions, i.e., temperature and time at temperature, and good ductility as well. In particular, alloys in accordance with the invention may be made which possess a minimum yield strength of 70,000 p.s.i. while retaining satisfactory magnetic properties.

In the production of alloys containing vanadium, cobalt and iron, the element carbon is usually regarded as an undesirable impurity because it is considered damaging to magnetic properties. Carbon concentration is usually reduced to the lowest level which is economically or technically feasible during melting and/or during annealing at a subsequent stage of solid state processing. 'It has now been discovered, however, that the deliberate increase of carbon concentration in alloys containing cobalt, iron and vanadium has the beneficial result of altering the recrystallized microstructure to effect a substantial increase in yield strength while producing little or no detriment to ductility and magnetic properties.

As presently melted and made available commercially, Vanadium Permendur contains less than 0.02% carbon and usually less than 0.01% carbon. I have found that when the carbon is increased to the range of 0.025 to 0.1%, the yield strength of the alloy is materially improved. Oarbon contents above about 0.1% result in relatively little additional refinement of recrystallized grain size and, hence, relatively little strength increase. The additional carbide particles formed by increasing the carbon concentration to greater than 0.1% have a disproportionately harmful etfect on magnetic properties particularly the alternating current properties. As the examples below indicate, in the fully recrystallized condition, the yield strength for Co-V-C-Fe alloys increases only slightly with increasing carbon content up to a threshold concentration of about .020% carbon. Yield strength then increases very rapidly with increasing carbon content up to a concentration in the range .025.030% carbon. As carbon is increased above this level yield strength again increases rather gradually. The yield strength level attained in the range .025.030% carbon is approximately 70,000 p.s.i.

The following examples will illustrate the practice of the invention and the critical effect of carbon on properties.

Alloys of the composition described in Table I were prepared in ingot form and reheated to 2250 F., thereafter hot rolled to 0.080-inch thickness. The resulting hot rolled strip was annealed at 1450 F. and brine quenched to render it suificiently ductile for cold rolling. It was determined metallographically that. the grain size of the quenched 0.080-inch strip containing 0.028% or more carbon was distinctly finer than that of the 0.0045 carbon material. After descaling by sand blasting, sample sections of the annealed and quenched 0.080inch strip from each of the four heats were cold rolled to 0.014" and 0.010" respectively, without intermediate heat treatment. This processing duplicated as nearly as possible the practices used in the commercial production of Vanadium Permendur.

TABLE I Go V Fe 49. 05 2. 05 B alance 48.80 2. 05 D0. 48. 95 2.07 Do. 48.90 2. 05 Do. 48.90 2 02 Do. 49.60 1. 84 D0.

Ring samples (1'' ID. X 1 /2" 0.1).) and tensile test specimens were prepared from the cold rolled experimental strip and similarly from the 0.014" and 0.010" strip, representing commercially available Vanadium Permendur (the latter designated heat 73205 and 82228 respectively). The 0.014" test samples were batch annealed (stacked laminations and tensile specimens in a welded box) at various temperatures in the range 1300 to 1550 F. and for various times in dry hydrogen (dew point of hydrogen supply was F. or dryer). The 0.010 samples were continuously annealed, singly and without stacking, in a belt furnace for 10 minutes at 1400 F., 1450 F. and 1500 F. in dry hydrogen atmosphere having a dew point of about 20 F.

The tensile and magnetic tests results are disclosed in Table II for the compositions described in Table I. The results with respect to samples continuously annealed are described in Table III.

TABLE II Tensile tests 13.0. permeability Core loss Percent elonga- H at H at B at B at B at B at B at 400 Hz. 400 Hz. 2,400 Hz. 2,400 Hz. Heat Anneal .2% Y.S. tion 20 kb 22 kb. H H H B 200 H 15 kb. 20 k 10 kb. 15 kb.

119, 300 8.0 50. 4 2, 080 16, 250 19,980 21, 240 21, 860 121. 9 230. 1 510 1, 107 120, 950 9. 5 38 138 5, 18, 100 20, 580 21,660 22,420 111 199 441 988 58, 330 5. 8 7. 14 23. 5 20, 570 21, 650 22, 560 22,880 23, 11 21. 1 38. 6 148 412 55,325 6. 8 6. 04 34. 0 20, 950 21,900 22,730 23, 770 23, 180 20. 4 34. 1 421 60,000 6.0 6. 16 38. 0 20, 810 21,820 22,400 22,850 23, 100 18.1 31. 4 149 394 62, 750 6. 8 6. 22 22. 4 20, 820 21,830 22, 650 22, 97 0 23,190 19.8 34. 8 144 428 40,595 5. 0 4. 88 18.1 250 22, 100 22, 780 22, 960 23, 130 16.8 30. 1 130 397 122, 400 9. 8 40 149 4, 280 17, 800 20,430 21, 510 22, 270 118 211 471 1, 051 126, 150 8. 8 35. 8 137 5, 530 18, 400 20, 580 21, 660 22, 380 113 204 457 1, 006 68, 930 5. 5 7. 8 27. 5 20, 470 21, 490 22, 460 22,820 23,040 24. 6 46.4 150 437 67, 875 6. 3 7. 9 33 20, 400 21, 450 22, 400 22, 760 23, 000 23. 4 40. 6 162 437 72, 875 6. 3 7. 0 27. 6 20,630 21,655 22, 590 22,930 23, 24. 8 42. 7 162 459 71, 800 5. 3 7. 7 30 20,530 21, 550 22, 480 22,870 23, 180 27. 5 50. 1 169 489 F., 4 50, 020 5. 5 6. 6 25. 6 20, 680 21, 750 22, 580 22, 970 23, 140 22. 5 42. 5 147 407 1,300: 2, 2 128, 150 7. 5 27. 8 105. 4 12, 070 19, 250 21, 020 21, 890 22, 530 77. 0 152.1 354 851 1,325 1 1,325 F., 2 hrs.- 74, 310 7.3 10.3 35.2 1.2 296 537 1,325 F., 4 hrs.. 73, 450 5. 8 8. 26 28.9 52.3 182 499 1,400 F., 1 hr 73,000 6.5 8.02 29.9 52.9 189 489 1,450 F 1 hr- 74, 150 6. 8 8. 68 31. 6 57. 4 181 505 1,550 F., 4 hrs-.-. 65,755 6.8 7.9 29.1 53.4 172 474 1,300 F., 2 hrs.- 124, 800 10. 0 38. 6 95.0 218 494 1, 104 1,325 F., 1 hr 1,325 F 2 hrs. 73, 115 7. 0 14.8 55 19,070 20, 550 21, 720 22, 280 22, 660 37. 0 74. 0 200 542 1,325 F 4 hrs. 70, 225 7. 0 10. 3 60. 0 19,900 21, 100 22, 280 22, 760 23,000 31. 8 58. 1 189 517 1,400 F 1 hr, 73, 350 5. 5 9. 54 35. 4 20, 075 21,255 22,285 22, 770 23, 140 32. 2 59. 2 195 509 1,450 F., 1 hr 71,525 5.3 10. 4 39 19,980 21,100 22,240 22,770 23,050 34.0 63.2 191 521 1,550 F 4 hrs. 61, 440 6. 0 9. 52 34 20,080 21, 260 22,350 22,800 22,990 31. 6 57. 7 173 479 1,300 F., 2 hrs. 110, 850 13. 0 44 168 2,010 17,280 20, 220 21,390 22,230 125. 3 214. 2 494 1, 054 325 F 1 hr, 107, 000 11. 5 33. 8 106 5,820 18, 540 20, 800 21, 900 22, 650 104 192 434 954 1,325 F., 2 hr 60, 830 6. 8 6. 58 25 20, 780 21,760 22, 550 22,810 23, 01 27.8 52. 5 220 619 1,325 F., 4 hrs. 58, 900 7. 8 7. 20 24 20,700 21, 760 22, 650 22,985 23, 120 24. 4 41. 6 193 539 73295..-. 1,400 F., 1 hr, 55, 425 8. 5 7. 24 25.2 20, 670 21,740 22, 640 22,940 23, 14. 9 25. 0 107 281 73205..-. 1,450 F., 1 hr. 57, 250 7. 5 5. 22 19. 4 21, 120 22,030 22, 730 22, 970 23, 190 18. 8 35. 4 146 437 73205.... 1,550 F., 4 hrs. 42,100 7. 0 5. 52 19. 7 21, 080 22,010 22, 650 22, 910 23, 060 22. 6 42. 0 181 520 TABLE III Tensile tests D.C. permeability Core loss Percent elonga- H at H at B at B at B at B at B at 400 Hz. 400 Hz. 2,400 Hz. 2,400 Hz. Heat Anneal .2% Y.S. tion 20 kb. 22 kb. 10 H 20 H 50 H 100 H 200 H 15 kb. 20 kb. 10 kb. 15 k 3835--. 1,400 F., 10 mins- 62, 500 8. 0 8.6 29. 2 20, 270 21, 440 22, 500 22,910 23, 120 16. 9 29. 1 109 261 3835..-. 1,450 F., 10 ruins- 58, 600 7. 3 8. 6 32 20, 310 21, 430 22,420 22, 730 22, 970 15. 8 24. 0 102 243 3835--. 1,500 F., 10 mins. 56, 800 7. 3 8. 54 33. 6 20, 260 21, 380 22,370 22, 730 22,940 14. 9 23. 2 101 237 3839..... 1,400 F., 10 mins.. 73, 650 7. 5 9. 74 37. 2 20, 210 21, 330 22,360 22,760 22, 960 22.0 36. 1 120 294 3839.... 1,450 F., 10mins- 71, 525 8.3 9. 48 38 20,110 21, 22,270 22, 690 22,930 21. 0 34. 4 115 280 3839.... 1,500 F., 10 mins. 69, 000 7. 5 9. 08 35. 6 20, 160 21, 230 22, 320 22, 690 22, 930 20.0 36. 1 118 281 3837.-.-. 1,400 F., 10 mins. 77, 875 8.0 11. 3 46. 4 19, 765 20, 930 22, 070 22,630 22, 865 26. 9 46. 2 139 327 3837.... 1,450 F., 10 mins- 74,000 7. 5 11. 2 46 19,760 20, 930 22,070 22,530 22,860 26.5 43. 6 133 314 3837..... 1,500 F., 10 ruins- 69,050 7. 5 11 46 19, 860 20, 930 22, 030 22,530 22, 810 26. 2 42. 0 131 311 3838... 1,400 F., 10 mins 76, 650 6.0 12. 6 49. 4 19, 515 20,730 22, 020 22, 640 22, 930 29. 7 55. 2 153 374 3838..-.- 1,450 F., 10 mins. 74, 300 7. 5 12.6 48. 6 19, 560 20,780 22, 020 22, 640 22, 950 28. 2 51. 4 141 342 3838....- 1,500 F., 10 ruins. 72, 875 6. 8 12. 6 48. 8 19, 560 20, 780 22,020 22, 580 22,870 28. 1 49. 3 140 334 82228... 1,400 F., 10mins- 55, 150 8.0 9. 3 34.0 20, 140 21, 275 22,335 22, 720 22, 990 16. 5 26. 2 105 262 82228.". 1,450 F., 10 mins 52,700 8. 8 9. 2 32. 6 20, 140 21,320 22,410 22,820 23, 090 15.6 20. 2 110 257 8 2228..- 1,500 F., 10 mins- 52, 300 7. 0 9.0 31. 8 20, 21, 380 22,410 22, 770 23, 000 15. 3 25.4 107 264 It was determined metallographically that recrystallization was incomplete in all of the samples annealed at 1300 F. for two hours and 1325 F. for one hour, and, although yield strengths in excess of 100,000 p.s.i. were developed, the corresponding magnetic properties were extremely poor (Table II). The 2-hour and 4-hour treatments at 1325 F. and all of the treatments at temperatures higher than 1325 F. resulted in complete recrystallization in all of the experimental heats and in the control material (Heat 73205). Each of the alloys containing 0.028% or more carbon developed yield strengths of 70,- 000 p.s.i. or higher (Table I) in one or more of the batch treatments which resulted in complete recrystallization. With one exception, the direct current magnetic properties, which are of primary importance in the 70,000 p.s.i. yield strength application, were within the limits generally specified for that strength level. In the recrystallizing treatments evaluated in these tests the heats containing less than 0.028% carbon developed somewhat better magnetic properties, but yield strengths were never higher than 63,000 p.s.i. The data in Tables II and III illustrate that, in the fully recrystallized condition, the materials containing 0.028% or more carbon have satisfactory and acceptable magnetic properties but at substantially higher strength levels than the materials containing less than 0.028% carbon. Similar structures and properties can be developed in short time heat treatments (10 minutes at temperature) by continuous annealing.

A useful increase in the yield strength of Vanadium Permendur can be obtained by a deliberate increase in the carbon content of the alloy. An increase in carbon concentration to higher than normal levels, i.e., greater than 0.020% carbon, results in the formation during processing of second phase particles, presumed to be vanadium carbides, which are then present in sufficient number to decrease significantly the recrystallized grain size developed during the annealing of cold rolled Vanadium Permendur strip or laminations and which efiectively limit grain growth following recrystallization. The action of the second phase particles is such that the development of a stable fine-grained recrystallized structure is relatively insensitive to variations in annealing time and temperature. The second phase particles and the associate fine grain size act in combination to produce yield strengths which are substantially higher than those developed in the normal alloy by the same annealing conditions. The ductility of the recrystallized fine-grained structure, as measured by elongation in a tensile test, is acceptable but somewhat lower than that of the normal alloy. Although the presence of second phase particles causes some deterioration in magnetic properties, the associated fine-grained structure is magnetically satisfactory.

An important advantage of the invention is that the yield strength improvement associated with increased carbon concentration in Vanadium Permendur is attained in fully recrystallized structures in which grain growth following recrystallization is relatively limited. These characteristics greatly increase the range of annealing conditions which can be employed to realize the strength improvements. It is difficult to attain a strength increase in the conventional alloy by establishing a partially recrystallized or barely recrystallized structure, particularly with a batch heat treatment. In contrast, the fully recrystallized grain structure is relatively stable and, therefore, the yield strength of the higher carbon alloys, e.g., 0.055% carbon alloy, Heat 3837, remains high. A low carbon aly (Heat 3835, .0045%C) and the normal commercial alloy (Heats 73205 and 82228) develop lower strengths in the fully recrystallized condition and show a greater tendency toward grain growth and loss of strength with increasing annealing temperature and time. The similarity of the yield strengths and magnetic properties developed by batch heat treatments (Table I) and by continuous annealing (Table II) further indicate the latitude in annealing conditions which may be employed to develop the strength improvements which are characteristic of alloys having controlled critical amounts of carbon in accordance with the invention. It should be noted that although the alloys described were relatively pure with regard to the concentrations of residual elements (Mn, P, S, Si, Cr, Ni, and Al), it is believed that strength levels may be increased still further by solid solution strengthening elfects resulting from an increase in the concentrations of one or more of the residual elements or by adjusting the base composition of the alloy. The strengthening effects of carbon can be superimposed on strength improvements attained by solid solution elfects.

It is apparent from the foregoing that various changes and modifications may be made without departing from the invention. Accordingly, the scope thereof should be limited only by the appended claims wherein what is claimed is:

1. An improved cold worked and subsequently fully recrystallized alloy consisting essentially of 47.5 to 50.5% cobalt, 1.7 to 2.1% vanadium, the balance essentially iron and containing 0.025 to 0.10% carbon and having a yield strength of at least 70,000 p.s.i.'

2. An improved alloy according to claim 1 having 0.03 to 0.08% carbon.

References Cited UNITED STATES PATENTS 3,024,141 3/1962 Burket et a1 148-3155 2,717,223 9/1955 Binstock et a1 148-122 RICHARD 0. DEAN, Primary Examiner US. Cl. X.R. 

